Some magnetic materials have anisotropy—a property whereby the materials are easier to magnetize along one axis, the easy axis, than in another, or hard, axis. Aniostropy is a result of many effects, including crystal structure, orientation of nanostructures of the material, stress and magnetostriction, and typically affects the shape of hysteresis loops, permeability and other magnetic properties of the material. It results in magnetic properties that differ with direction of applied magnetic fields.
Thin-film magnetic materials are attractive for micro-fabricated very-high-frequency (VHF) power magnetic devices, but their anisotropy makes them difficult to use in toroidal geometries, in which flux direction varies. Toroidal geometries are often attractive for use in power conversion applications because they emit little external magnetic field and, by keeping flux in the plane of the core, help reduce eddy current losses compared to other inductor geometries, such as solenoids and planar spirals. External magnetic fields are typically undesirable because they may cause electromagnetic interference and they may induce eddy-currents in nearby conductors and shielding materials, potentially leading to power losses.